This invention relates to a circuit arrangement for operating a discharge lamp, comprising a DC-AC converter provided with
a branch A having ends suitable for connection to a DC voltage source and comprising a series circuit of two switching elements for generating a periodic voltage by being conducting and non-conducting alternately at a frequency f, each switching element being shunted by a diode, PA1 a control circuit coupled to control electrodes of the switching elements for rendering the switching elements conducting and non-conducting alternately at the frequency f, PA1 a load branch B which shunts one of the switching elements and which comprises inductive means L and means for coupling the discharge lamp to the load branch B, and PA1 means M for adjusting the power consumed by the discharge lamp. PA1 means Mp for adjusting the value of the difference Tt-Td, in which Tt is a time interval during which one of the switching elements is conducting during a half cycle of the periodic voltage, and Td is a time interval during which a diode is conducting during this same half cycle of the periodic voltage. PA1 means for generating a signal S1 which is a measure of Tt-Td, PA1 means for generating a signal S2 which is a measure for a desired value of Tt-Td, and PA1 means for rendering the signal S1 substantially equal to the signal S2. PA1 means for generating a signal P1 which is a measure of the power consumed by the discharge lamp, and PA1 means for generating a signal P2 which is a measure of a desired value of the power consumed by the discharge lamp.
Such a circuit is known from European Patent 323676, which corresponds to U.S. Pat. No. 4,949,016. In this Patent, both the frequency f of the DC-AC converter and the time interval Tt during which each of the switching elements is conducting are presented as parameters by which it is possible to adjust the power consumed by the lamp. It was found that the use of one of these parameters makes it possible to adjust the luminous flux of the lamp over a wide range by comparatively simple electronic auxiliary means. A disadvantage which may arise when the frequency f is used as the parameter is that the relation between the power consumed by the discharge lamp and the frequency f is not unequivocal over the entire range of frequencies which can be set. Especially when the power consumed by the discharge lamp is comparatively low, each value of the frequency f in a certain range of this frequency f can correspond to two lamp power values. This results in an unstable burning of the lamp. It was found that for very many discharge lamps in practice, especially compact fluorescent lamps, it is not possible for this reason to adjust comparatively low values of the power consumed by the discharge lamp. In other words, the range over which the discharge lamp can be dimmed is limited.
It should be noted that European Patent 482705, which corresponds to U.S. Pat. No. 5,198,726, describes a possible solution to this problem. This solution, however, is comparatively complicated and expensive.
An important disadvantage connected with the use of the time interval Tt as a parameter is that, depending on the dimensions of the discharge lamp, the power consumed by the discharge lamp is a very steep function of the time interval Tt in a certain range. This means in practice that additional control measures are necessary for adjusting the power consumed by the discharge lamp by means of the time interval Tt in this range. These additional control means also render the use of this parameter comparatively complicated and expensive.